Velocity Profiles Around Hoods and Slots and the Effects of an Adjacent Plane

Fletcher, B.; Johnson, A. E.

doi:10.1093/annhyg/25.4.365

Cited by 14 publications

(9 citation statements)

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“…2). The gradient of the regression line was approximated as -1, and the centerline velocity equation for the circular slot hood examined was derived as follows: Further, the area of the slot (A) is the product of W multiplied by the circumference of the slot (L=1.57 m), and the rate of airflow into the hood (Q) is then calculated by means of the (4). For example, the recommended flow rate for the circular slot hood will be 20.56 m3/min when the capture point is located at a point of 150 mm from the opening, and the capture velocity is estimated at 0.45 m/sec.…”

Section: Characteristics Of Centerline Velocities In a Circular Slot mentioning

confidence: 99%

Design of a Circular Slot Hood for a Local Exhaust System and its Application to a Mixing Process for Fine Particles and Organic Solvents.

Iwasaki

Ojima

1997

INDUSTRIAL HEALTH

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The characteristics of airflow (pressure loss and entry loss factor) were measured around a circular slot hood for its application to a local exhaust system. Centerline velocity, defined as the ratio of air velocity on the centerline of the slot hood to average slot face velocity, was found to be independent of the airflow rate. The relationship between the centerline velocity and the ratio of centerline distance to slot width was also found to be independent of the slot size. The empirical centerline velocity equation for the circular slot hood was thus constructed to design the local exhaust system. Recommended values for airflow rate into the circular slot hood and the average slot face velocity were found to be 20.14 m3/min and 8.55 m/sec, respectively. The optimum air velocity at a capture point was also found to be 5 % of the average slot face velocity, i.e. 0.43 m/sec, and the effective ventilation with the hood was achieved with these values. The local exhaust system with the circular slot hood was installed for a mixing process of fine particles and organic solvents in a magnetic coating works. The effectiveness of the circular slot hood was confirmed by measuring the concentrations of airborne particles and vapors before and during the operation of the local exhaust system.

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Section: Characteristics Of Centerline Velocities In a Circular Slot mentioning

confidence: 99%

Design of a Circular Slot Hood for a Local Exhaust System and its Application to a Mixing Process for Fine Particles and Organic Solvents.

Iwasaki

Ojima

1997

INDUSTRIAL HEALTH

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“…Past research on local exhaust ventilation (LEV) has relied mainly on the studies by DallaVaUe (6,7,8) and Silverman(27,28). Fletcher (12)(13)(14)(15)(16) and GaiTison(20,21) expanded these works. Fletcher (13) experimentally measured the variation in velocity between hoods with different width to length ratios.…”

Section: Literature Reviewmentioning

confidence: 99%

“…Lastiy, the predicted velocity at the center of the hood face is 64% of the actual face velocity. 16 The potential function for flow through an elliptical space was found to be: The major drawback of this approach is that "velocity contours generated by multiple openings are not additive in the simple sense(ll)". Also, the velocity predicted by the model went to infinity at the center of the hood face opening when Z=0.…”

Section: Conservation Of Massmentioning

confidence: 99%

A Comparison of Three-Dimensional Velocity Models for Flanged Rectangular Hoods

Flynn

Fitzgerald

1989

Applied Industrial Hygiene

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Marie Louise Fitzgerald. Comparison of Three-Dimensional Velocity Models for Flanged Rectangular Hoods. (Under the direction of Dr. Michael R. Flynn)The purpose of a local exhaust hood is to capture an airborne contaminant as close to the point of generation as possible and prevent it from entering the worker's breathing zone. Presently, local exhaust hood designs are based on capture velocity and practical experience. This process involves several defiencies, the most significant being the inability to determine the contaminant concentration in the worker's breathing zone as well as the inability to incorporate crossdrafts and source momentum characteristics.Three-dimensional velocity field models which predict the magnitude and direction of the velocity vector at any point in the field of flanged, rectangular exhaust hoods have been developed recently. This is a first step in determining the contaminant concentration in the worker's breathing zone and can, by vector addition, predict how crossdrafts and source momentum will effect contaminant capture.The purpose is to determine which of the available velocity field equations is the most accurate predictor of the velocity vector. This should help to determine optimal hood size, position and flow, resulting in more efficient and economical exhaust designs for industry. The theoretical basis for models in potential theory is presented and the limitations for industrial settings are discussed.

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“…Nearly all of the relevant scientific literature published regarding the effectiveness of capturing hoods (Silverman, 1942;Fletcher B. , 1977;Fletcher B. , 1978;Garrison, 1981;Fletcher & Johson, 1982;Flynn & Ellenbecker, 1987;Conroy, Ellenbecker, & Flynn, 1988) have employed the concept of "capture velocity", which is defined by ACGIH as "the minimum hood-induced air velocity necessary to capture and convey the contaminant into the hood" (ACGIH, 2007 , 2003). For capturing hoods Flynn and Ellenbecker (1986) argued that capture velocity is an inadequate surrogate measure of capturing hood performance.…”

Section: Velocities In Front Of a Capturing Hoodmentioning

confidence: 99%

Effectiveness of a Flanged and Unflanged Small Rectangular Capturing Hood

Kasberger¹

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Capturing hoods are an important tool for protecting workers from hazardous airborne exposures and often preferred because their small sizes offers little interference to the worker or process. Current ventilation guidelines for capturing hood assume that exceeding a recommended capture velocity ensures an acceptable degree of effectiveness. That assumption has little basis in scientific studies. This study investigated the effectiveness of a 6 inch by 12 inch rectangular capturing hood centered on a work table with a manikin acting as a surrogate for a worker. The study utilized a tracer gas (Freon-134a) to evaluate effectiveness of a capturing hood at three different hood airflows (Q), two different cross draft velocities (V cross), and with the presence or absence of a flange (Flange). The tested air flows through the hood produced velocities at the leading edge of the source (V x) of approximately 40, 100, and 140 fpm, as measured with a constant temperature anemometer. The study was conducted inside of a 9 ft high, 12 ft wide, 50 ft long wind tunnel with the manikin's back to the cross draft. Tracer gas was released from a custom made source located 11" in front of the capturing hood. The manikin was heated and "breathed." Its hands were positioned on either side of the source. Breathing zone samples from the nose (C nose) and mouth (C mouth) of the manikin were collected simultaneously to evaluate the protection efficiency of the hood. Samples were also taken simultaneously from inside the duct (C duct) to evaluate the capture efficiency of the hood. Samples were collected in 5 liter Tedlar® bags at a rate of 0.2 lpm over a period of 20 minutes for each sample. Sample bag contents were analyzed by a Gastec FT-IR. The study used a randomized factorial design with two replications. The results showed that the capturing hood was surprisingly highly effective both in terms of minimizing manikin exposures and minimizing escape of contaminant to the room environment. The concentration of Freon measured in the breathing zone was less than 1 ppm for all tests and mostly was below 0.2 ppm, even though the source concentration was over 370,000 ppm and was released at 1.79 lpm. This was true even when the capture velocity was only 50 fpm and the cross draft velocity was 60 fpm. The capture effectiveness of the hood was no less than 90% but the results may not have been reliable due to apparent measurement inaccuracies and imprecision at the Freon levels measured. The results only apply to this hood under these conditions. Testing under broader ranges of conditions would be necessary before concluding that the capturing hood is always highly effective. iii ACKNOWLEDGEMENTS This project was supported by the National Institute of Safety and Health through their training grant number: 5 T01 OH008431. I am deeply indebted to my academic advisor, chair, and mentor Dr. Steven Guffey, not only for his guidance, advice, and support through the entire thesis process, but for the guidance and advice he has imparted to me regardi...

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Velocity Profiles Around Hoods and Slots and the Effects of an Adjacent Plane

Cited by 14 publications

References 0 publications

Design of a Circular Slot Hood for a Local Exhaust System and its Application to a Mixing Process for Fine Particles and Organic Solvents.

Design of a Circular Slot Hood for a Local Exhaust System and its Application to a Mixing Process for Fine Particles and Organic Solvents.

A Comparison of Three-Dimensional Velocity Models for Flanged Rectangular Hoods

Effectiveness of a Flanged and Unflanged Small Rectangular Capturing Hood

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